I thought she was only presented with the theory of her death. I also thought the ''17 people'' part was talking about Kanon, not about her.

If they were talking about Erika, it was most likely just to confuse her and deny her existence. Since the scene was blasted by explosives, no evidence suggested she was alive, so she had no way to counter it.

It's basically like in the early stages of Umineko, where Battler keeps trying to deny witches, he used the same logic on Erika, where the theory was used as a weapon against her to kill her rather than a fact.

Just like Bernkastel showing that Lion would be shot by Kyrie.

I think that the red truth saying ''There are only 17 people'' related to Kanon(or someone of the Yasu-tree) and not to Erika, because I don't think saying Erika was dead would expose any of Beatrice's heart, it was made clear that saying that truth would compromise the secret of Beatrice, and Erika has nothing to do with that.